Rapid Variability of Blazar 3C 279 during Flaring States in 2013-2014 with Joint Fermi-LAT, NuSTAR, Swift, and Ground-Based Multi-wavelength Observations

Abstract

We report the results of a multi-band observing campaign on the famous blazar 3C 279 conducted during a phase of increased activity from 2013 December to 2014 April, including first observations of it with NuSTAR. The $\gamma$-ray emission of the source measured by Fermi-LAT showed multiple distinct flares reaching the highest flux level measured in this object since the beginning of the Fermi mission, with $F(E>100\mathrm{MeV})$ of $10^{-5}$ photons cm${}^{-2}$ s${}^{-1}$, and with a flux doubling time scale as short as 2 hours. The $\gamma$-ray spectrum during one of the flares was very hard, with an index of ${\Gamma }_{\gamma }=1.7\pm 0.1$, which is rarely seen in flat spectrum radio quasars. The lack of concurrent optical variability implies a very high Compton dominance parameter $L_{\gamma }/L_{\mathrm{syn}}>300$. Two 1-day NuSTAR observations with accompanying Swift pointings were separated by 2 weeks, probing different levels of source activity. While the 0.5$-$70 keV X-ray spectrum obtained during the first pointing, and fitted jointly with Swift-XRT is well-described by a simple power law, the second joint observation showed an unusual spectral structure: the spectrum softens by $\Delta {\Gamma }_{\mathrm{X}}\simeq 0.4$ at $\sim$4 keV. Modeling the broad-band SED during this flare with the standard synchrotron plus inverse Compton model requires: (1) the location of the $\gamma$-ray emitting region is comparable with the broad line region radius, (2) a very hard electron energy distribution index $p\simeq 1$, (3) total jet power significantly exceeding the accretion disk luminosity $L_{\mathrm{j}}/L_{\mathrm{d}}\gtrsim 10$, and (4) extremely low jet magnetization with $L_{\mathrm{B}}/L_{\mathrm{j}}\lesssim 10^{-4}$. We also find that single-zone models that match the observed $\gamma$-ray and optical spectra cannot satisfactorily explain the production of X-ray emission.

17 pages, 10 figures, accepted for publication in The Astrophysical Journal

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